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Previously, ParentLocalSurfaceIdAllocator would default to an invalid
LocalSurfaceId. A new LSI had to be explicitly allocated before the
normal behavior could use it.

With ParentLocalSurfaceIdAllocator now default initializing a valid LSI,
the various call sites no longer need to explicitly check for the
uninitialized state.

Render Widget is one of those call sites.

BUG=837030

Change-Id: I330d2c47492f1ac577a928f20a8e8e4e14f91b29
Reviewed-on: https://chromium-review.googlesource.com/1038127
Commit-Queue: Chris Blume <cblume@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585361}